Ancient Rocks, Modern Purpose

In the desert nation of Oman, a mass of rock from Earth’s mantle has been thrust to the surface to form some very unusual mountains. The formation, the Samail Ophiolite, has long fascinated geologists because it exposes rocks normally inaccessible in the deep Earth. Recently, there has been a surge in interest in these rocks, as scientists have recognized that such formations may naturally take up and store large amounts of carbon, and thus play a previously unrecognized role in regulating global climate.

In Oman, geologist Peter Kelemen is investigating unusual rocks thrust up from the deep earth for their potential to store human carbon-dioxide emissions.

Geologists have long recognized that these processes take place. But new studies by geochemist Peter Kelemen and colleagues have revealed that the reactions are far more rapid and prevalent than previously thought. Scientists had assumed that most visible carbonates formed long ago. However, by sampling rocks from road cuts, wells, boreholes and other sources, Kelemen and former Lamont geochemist Jürg Matter have found that many carbonate veins here actually formed during human time, and that the process is ongoing and fast. They also found that the reactions take place much deeper underground than previously thought, well below the surface at sites where CO₂ travels in groundwater through cracks.

Some underground reactions are so vigorous, they appear to create small explosions—a sort of natural hydrofracturing—that open more cracks, creating space for more reactions, and thus feeding a continuous loop of CO₂ uptake. Kelemen and Matter estimate that each cubic kilometer of peridotite naturally absorbs on average a ton of atmospheric carbon each year: up to 100,000 tons annually in the region. In places, the reaction has completely run its course; the rocks are fractured down to the tiniest pore spaces and filled with carbonate, creating entire mountains of the material. Theoretically, they say, there is enough peridotite in Oman and the neighboring United Arab Emirates to absorb 33 trillion tons of CO₂—1,000 years of human output, if present-day emission rates remained unchanged.

With CO₂ concentrations in the atmosphere recently measured at 400 parts per million, probably for the first time in some 3 million years, and little movement toward reducing industrial emissions, many scientists are now looking at ways to remove the gas from the air.

No one thinks that Oman’s rocks could solve the entire problem. But with some simple engineering, Kelemen and Matter think natural processes could be sped up a million times in places, and that accelerated rate could take up some of the manmade CO₂. One way to do so would be to drill boreholes and pump down heated water impregnated with CO₂; heat greatly speeds the reaction, and once jump-started this way, the cycle of natural cracking would do the rest, rapidly taking in any more CO₂ pumped down.

Oman is rich in oil and burns considerable oil in its power plants, so much of the CO₂ it produces could be delivered from the plants in pipelines. Another method would take CO₂ directly from the air. This step would involve pumping surface seawater, which naturally absorbs atmospheric CO₂, down pipes into the mantle rocks just offshore. The cleaned-up water would then be flushed back to the sea surface, where it would then absorb more CO₂. Accounting for engineering challenges and the obviously large expense, the researchers assert that Oman’s peridotite still might be harnessed to absorb as much 4 billion of the 30 billion tons of the atmospheric CO₂ currently produced annually by humans.

“I’m not convinced that society is really going to do very much about [climate change] until we find out what the long-term effects are going to be,” Kelemen said. “But at that time, there might be renewed interest in how you might get CO₂ out of the air. We’re trying to develop methods so that when and if we do get ready to use them, they’ll be available.”